A Light Emitting Diode (LED) is a solid state device that converts electrical energy to light. Light is emitted from an active layer of semiconductor materials sandwiched between oppositely doped layers when a voltage is applied across the doped layers. The efficiency of an LED structure at converting energy to light determines whether the LED is suitable for certain applications. For example, use of LEDs in lighting applications requires high efficiency, reliability, and low cost. Advances in semiconductor materials and improvement in LED architectures have led to improvement in efficiency.
U.S. Pat. No. 6,121,635 to Watanabe discloses a current blocking layer positioned below a top electrode for increasing the luminous efficiency of the LED. Because the current blocking layer is below the top electrode, is light transparent, and extends beyond the perimeter of the top electrode, the current blocking layer prevents high current density in a region of the LED where any light emitted would be blocked by the nontransparent top electrode. The disclosure of Watanabe indicates that increased efficiency is achieved by preventing emission of light under the nontransparent electrode. The current is directed elsewhere so that the resulting generated light can escape the device. U.S. Pat. No. 7,247,985 to Koneko similarly suggests improved electrical energy conversion by providing two current blocking structures in an LED. A first current blocking structure is disposed directly under the top electrode in a central region. A second current blocking structure is disposed in an outer region surrounding the central region. The second region functions to define the shape of the light emitting region and Koneko says improved light-emission performance is achieved. The use of one current blocking structure or a pair of current blocking structures as disclosed in these patents may create some gains in efficiency but these prior art structures also have limitations.
FIG. 1 (Prior Art) is a cross-sectional side view of a conventional lateral LED device 1. Lateral LED device 1 includes a bond wire 2, p-electrode 3, Indium Tin Oxide (“ITO”) transparent conductive layer 4, current blocking layer 5, p++GaN layer 6, p-GaN layer 7, active layer 8, n-GaN layer 9, growth substrate layer 10, n-electrode 11 and regions of non-uniform light generation 12. P-electrode 3 and n-electrode 11 are non-transparent metal layers. During operation of lateral LED device 1, a voltage is placed across p-electrode 3 and n-electrode 11 of lateral LED device 1 causing a current to flow from p-electrode 3 to n-electrode 11. This flow of current causes light to be generated in active layer 8. Current blocking layer 5 is a transparent insulating layer and is disposed between p-electrode 3 and light-emitting active layer 8 to prevent the emission of light under non-transparent metal p-electrode 3. Current blocking layer 5 thus prevents current flow and light emission in a portion of active layer 8 where overlying metal p-electrode 3 would obstruct the emitted light. Current flow is thus directed to other portions of active layer 8 which increases the luminous efficiency of the device. Because the resistance of ITO layer 4 and p-GaN layer 7 is higher than n-GaN layer 9, the current flowing from p-electrode 3 through n-GaN layer 9 tends to be concentrated at the edge of current blocking layer 5 nearest p-electrode 3. Current flow farther from p-electrode 3 is less dense and this disparity leads to non-uniform light generation 12, local heating from the concentration of electrical currents, and potential damage to lateral LED device 1.
FIG. 2 (Prior Art) is a cross-sectional side view of a conventional vertical LED device 20. Vertical LED device 20 includes: n-electrode 21, n-GaN layer 22, active layer 23, p-GaN layer 24, p++GaN layer 25, current blocking layer 26, highly reflective layer 27, encapsulant layer 28, barrier metal 29, bond metal layer 30, adhesion layer 31, conductive carrier 32, p-electrode 34, and regions of non-uniform light generation 35. During operation of the vertical LED device 20, a voltage is placed across the device such that current flows from metal p-electrode 34 to metal n-electrode 21. As current flows through active layer 23, light is generated. Current blocking layer 26 is disposed between p-electrode 34 and active layer 23 to prevent current flow and light emission under the non-transparent metal n-electrode 21. The highly reflective layer 27 is highly conductive, so the entire p++GaN region 25 to the right of current blocking layer 26 is essentially equipotential. The overlying n-GaN layer 22, however, is somewhat resistive and limits current spreading. Current density in regions closer to n-electrode 21 is therefore greater than current density in regions farther away from n-electrode 21. This disparity in current density causes non-uniform light generation 35 in vertical LED device 20. Moreover, the high current density closest to n-electrode 21 may cause local heating and damage to LED device 20. An LED device with improved luminous efficiency and uniform light generation is desired.